The list quite long so I will keep myself short here:
# Recipes:
We have changed or improved some recipes add others and remove some more. Most recipes can be found in guidebook so I wont go over the changes. The enchanter repair recipes are the only worth while thing going over.
For starters they ahve to be enabled in the config and only repair dungeon exclusive loot.
# Blocks:
We have added alot of new blocks. Most of those blocks are decorational. Some blocks were added to keep in parity with bta, like mossy holystone. Other just made sence to add such as the different colored skyroot planks, dungeon chests.
# Mobs
Most mobs stayed mostly the same, with slight adjustment to their spawning. Boss fight were tweaked to adjust to some of the bigger changes to items. Mimic has gotten a glowup. There are to many features to list here and I am scared of Wallace.
# Item
By far the biggest changes are to items. For one armor has unlike beta minecraft resistances to different damage types. To give each armor a use and we added additional damages types, lightning and holy damage. A good chunk of weapons and tool now also does different types of damage. For example gravitite tool do fall damage. Another huge changes is to accessories. The original Aether had a lot of slot and very little to fill them. We removed rings a dedicated shield slot and the wildcard slots. We have expended the accessories. Most of them have some effects and render on the player. Additionaly the trinket slot the 2 last slots can hold a huge selection of items, including some vanilla items. If you are currious if an item can be equipped in those slot you can hover over them. If the icon changes to the outline of said item than it can be equipped.
There is much more. You can, hopefully soon, read up on them on the official
Better with Aether wiki